This invention relates to wireless communication systems and, more particularly, to a wireless communication system using modulated backscatter technology.
A variety of wireless communication technologies are now being utilized in Electronic Shelf Label (ESL) systems to convey pricing information of the items for sale in a supermarket. These systems are arranged to electronically change the price on ESL tags to replace the time-consuming process of changing prices on paper labels. The price displayed on the ESL tags is updated using a highly reliable xe2x80x9cdownlinkxe2x80x9d communications path from the ESL system controller to the ESL tag to assure that the price displayed on the ESL tag is identical to the price stored in the supermarket""s computer database. This communication path is called the xe2x80x9cdownlinkxe2x80x9d, because it typically originates from a centralized unit mounted in the ceiling (ceiling node). To minimize pricing errors, such ESL systems must have a positive assurance that each ESL tag received a particular price change message. The ESL tags transmit a positive acknowledgement over an xe2x80x9cuplinkxe2x80x9d communication path to the ceiling node. Not only is it essential for the uplink to be reliable, but for ESL systems to be cost-effective, ESL tags must be very inexpensive and the supermarket infrastructure cost (the quantity and cost of the ceiling nodes) must be kept to a minimum. Thus, a problem exists to develop a reliable uplink communication path for an ESL system that is inexpensive yet has as great a communication range as possible.
In accordance with the present invention, a communication system (e.g., an ESL system) improves the uplink communication range, illustratively for an ESL tag, by sending a Continuous-Wave (CW) signal, at frequency f1 to the ESL tag, and by having the ESL tag generate a reflected radio signal (f1xc2x1f2) which has been modulated using a precise-frequency subcarrier signal f2. The subcarrier signal itself is modulated using an uplink information signal. The ceiling node, of our illustrative ESL system, receives and detects the modulated reflected radio signal and uses a narrowband filter to retrieve the modulated subcarrier signal which is then demodulated to obtain the uplink information signal. According to one aspect of the invention, a crystal oscillator is divided-down to produce the precise subcarrier frequency. According to another aspect of the invention, a homodyne receiver is used to detect the modulated reflected radio signal which is narrowband filtered to obtain the modulated subcarrier signal.